linux-kernel - Re: [PATCH block:for-3.3/core] block: disable ELEVATOR_INSERT_SORT

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Date:	Thu, 5 Jan 2012 20:38:07 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH block:for-3.3/core] block: disable ELEVATOR_INSERT_SORT_MERGE

Hello,

On Thu, Jan 5, 2012 at 8:19 PM, Shaohua Li <shaohua.li@...el.com> wrote:
>> For detailed discussion of the bug:
>>
>>  http://thread.gmane.org/gmane.linux.kernel.next/20064/focus=20159
>>
> this is overkill. when plug is added, we found huge performance
> regression, that's why we add INSERT_SORT_MERGE.

With what workload? I suspect most of the improvements is from merging
across different cfqqs, no? The whole recursive thing can't be very
useful if cross-cfqq merging isn't allowed. Maybe there are specific
cases where last_merge hint merging can be specially effective. I
don't know. Regardless, this is an apparent bug and the block tree
will be pushed mainline pretty soon. If necessary, fix it better
later. Doing complex things inside merge window usually isn't a good
idea.

Thanks.

-- 
tejun
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